D. c.-a. c. converter



July 15, 1958 v E. A. M. KITTL D. C.A. C. CONVERTER Filed May 4, 1954F|G.l

LOAD

FIG.2

IN VEN TOR.

EMIL AM.KITTL 5) I62 LOAD' A TTOR/VEY 11). C.-A. C. CUNVERTER Emil A. M.Kittl, Elheron, N. 1., assignor to the United States of America asrepresented by the Secretary of the Army Application May 4, M54, SerialN 0. 427,681

llti Claims. (Cl. Hit-49) (Granted under Title 35, U. S. Code (1952),see. 266) The invention described herein may be manufactured and used byor for the Government for governmental purposes, without the payment ofany royalty thereon.

This invention relates to converters, and more particularly toelectromechanical means for converting direct current toalternatingcurrent.

In many military and civil applications there is a demand foralternating'current power where the only reliable source of power isdirect current from storage batteries. There are several direct-currentto alternatingcurrent converters that have been developed to solve thisproblem, such as the interrupter type vibrators used in small carradios, dynamotors or electronic type oscillators. Each of these hasadvantages, but for use where power ratings of several hundred watts arerequired they all have low efiiciecny, short life, and are unreliablefor critical purposes. Furthermore, frequency and voltage outputcontrol, if required, involve complex additions.

Inverters which are based on the principle used in mechanical rectifiershave considerably better performance than the other types, but up untilnow they have been developed to work with a rigid and stablealternating-current power system and cannot be used with only adirectcurrent power source. In these mechanical inverters the directcurrent is switched through a transformer wtih successive inversions ofvoltage polarity which produce, on the secondary of the transformer, analternating voltage whose frequency will be the frequency of thesuccessive inversions.

This type of inverter, while potentially capable of carrying heavycurrents and giving substantial outputs over a wide range offrequencies, has several inherent disadvantages. Primarily, itsswitching mechanism must be driven by some external means such as asmall motor, an oscillating circuit or a mechanical vibrating device,all of which produce a rigid frequency that does not necessarilycoincide with the load requirements or other considerations for optimumoperation. This further aggravates the serious problem, which is that ofcurrent fiow through the switching contacts. The contacts may readilycarry the heavy load current of a hundred amperes when contact is firmlyestablished. However, if the current is permitted to begin flowingheavily at the instant of contact when the contact resistance isinherently high, a terrific heat and arcing will result to melt thesurfaces of the contact, to transfer material to the negative contactaccording to the Thompson effect, and otherwise seriously damage thecontacts if not render the device inoperative.

This latter problem may be alleviated to some extent by the use ofcapacitive and resistive networks across the contacts or by theinclusion of saturable reactors in series with the contacts. Suchreactors in a non-saturated state provide a very high impedance to theflow of the current at the instant of contact and until the currentthrough the reactor becomes great enough to saturate the core, at whichtime a minimum impedance is seen and a large current can flow throughthe contacts. The reverse is true at the opening of the contact. Thesesaturable reactors may also have a preexcitation winding which willcontrol the amount of flux and the length of time it takes to saturatethe reactor. This length of time may be changed by means of a variableresistor in the pro-excitation circuit.

Two features required in all of these mechanical inverters are thatthere must be an overlap of the closing of the alternate contacts andthat a capacitor must be provided across the load winding to permit asmooth transfer of voltage from one polarity to the reverse polarity byproviding commutating voltage in the closed loop of the primary circuitduring the time interval when both contacts are closed. The capacitor isnot technically used for filtering purposes.

Optimum conditions of the three elements of the converter are necessaryduring make or break to provide a maximum efliciency and the longestpossible life of the equipment. The commutating capacitor, the saturablereactors and the contact overlap time must be precisely established ifoptimum conditions are to be achieved.

In operation, the closing of the second contact on the other side of thetransformer primary winding short circuits the voltage across the wholeprimary, which is twice that of the supply voltage due toauto-transformer action. However, the current allowed to flow over thesecond contact at the instant of closing is very small due to the highimpedance of the saturable reactor which absorbs the high voltage andprovides a short time of very low current flow called a make step duringthe contact closing interval of the switch. This current make stepprevents damage to the surface of the contacts.

As soon as the reactor is saturated, the transformer primary isefiectively shorted, and if it were not for the commutating capacitorthe voltage would drop to zero before satisfactory current commutationcould be achieved. As the current drops in the first side of thetransformer through the first pair of contacts and saturable reactor,the reactor begins to desaturate, and under the influence ofpremagnetizing current soon provides a high impedance in series with thecontacts to create a so-called break step in the current during thecontact opening interval. During this low current break step and withthe aid of conventional arc-suppression techniques across the openingcontacts, if desired, these contacts can be opened with a minimum ofdamage at substantially zero current and voltage. When this firstcontact is opened the current of the second contact increases to a peak.A substantial current flows until the end of the conducting period whenthe cycle is repeated.

While the basic frequency which is established by the contact drivingmechanism is not critical, there are very critical specificationsregarding the overlap time between the two sets of contacts in relationto the process of current commutation. The best solution to this problemof matching the overlap time and the requirements for the currentcommutation is the use of a new magnetic switch which, under goodconditions, has a lifetime of about a billion operations and a responsetime of about .0001 second with a current-carrying capacity as high asamperes.

It is therefore an object of this invention to provide for the use ofmagnetic switches in mechanical inverter circuits.

It is a further object of this invention to provide a mechanicalinverter having ideal parameters in regard to current commutation.

Other and further objects of this invention will become apparent fromthe following specifications and the drawings of which Figure 1 shows atypical embodiment of this invention using a single magnetic switchingdrive and Figure 2 shows an embodment using a push-pull magneticswitching drive.

Referring now to Figure 1, a direct current is applied across terminalsand 12 with terminal 12 connected to the center tap 14 of transformer 16through smoothing choke 18. The current passes through either side ofthe transformer depending on the switching arrangement at that instantto pass through one of the saturable reactors to terminal 10. Thesesaturable reactors 2i} and 21 have windings 22 and 23 in series withcontacts 24 and 25 of their respective magnetic switches 26 and 27.These magnetic switches have first holding coils 28 and 29 alsoconnected in series with the contacts 24 and 25 to opposing sides 30 and31 of the push-pull input winding of transformer 16.

Windings 34 and 35 of reactors 2i) and 21 are connected in seriesthrough smoothing choke 38 and control resistor 40 across the powersupply to provide an equal premagnetization in these reactors.

Additional coils 42 and 43 in the magnetic switch are connected acrossthe input winding of the transformer 16 with rectifiers suitablyoriented to provide current only when the corresponding switch is closedand to provide a holding force actuated by the voltage when the loadcurrent is insutficient to provide a strong enough holding field throughthe series windings 28 and 29.

The actual switching in this invention is provided through the thirdwindings 46 and 47 of the magnetic switches. These windings areconnected in Series with rectifiers 66 and 67 and windings 48 and 49 ofsaturable reactors 50 and 51 across the primary of transformer 16.Additional windings 52 and 53 are provided in series with inductor 56and variable resistor 58 across the direct-current power supply forreasons to be explained later.

The alternating-current output is taken across terminals 6t? and 61 ofthe secondary of transformer 16 to any desired load 62. Comrnutatingcapacitor 64 must also be connected across secondary terminals 60 and61.

in operation, at closing of the switch 26 to a Voltage twice that acrossterminals 10 and 12, generated due to auto-transformer action across theprimary windings terminals 3t and 31 of transformer 16, appears acrossthe contacts 24 of switch 26. However, due to the high impedance ofwinding 22 of reactor 20 the voltage appears across this winding withnegligible voltage appearing at the contacts of the switch. This permitsonly a negligible current to flow for the interval of time necessary tomake the contact and such interval can be controlled by the choice ofreactor and pre-saturating currents through winding 34 which arecontrolled by variable resistor 4-1). These values can be adjusted tomeet the requirements and mechanical closing characteristics of anyparticular switch.

After the saturating interval or make step, the impedance of thesaturable reactor becomes effectively zero and the input voltage appearsacross the half primary winding terminals 30 and 14. The rate of changeof current between half primary windings 31 and 14 and 3d and 14 iscontrolled by the commutating capacitor 64 which permits only a gradualchange instead of an instant short-circuit across terminals 30 and 31.The decreasing voltage across terminals 31 and 14 causes the currentthrough the other path to decrease and as this current approaches acertain minimum value, which is determined by the pre-excitation of thereactor 21, the impedance of this reactor increases very suddenly toprovide an interval or break step of minimum current and voltage acrossthe contacts 25 of switch 27. At this instant the magnetic holding forceof the current in coil 29 disappears and at the same time the current inholding coil 1-3, which was dependent upon the voltage across theterminals 30 and 31, also decreases and before this voltage reaches zerothe totalmagnetic force produced by coils 29 and 43 becomes insufficientto overcome the force of the contact spring 71 tending to open theswitch. Contact spring 71 now opens the switch without damage since theinterrupted current is very small. If desirable, additional compensatingcircuits of well known types may be added to reduce the danger of arcingstill further.

In this circuit the timing for the frequency of the alternating-currentoutput is developed within the circuit rather than being imposed on thecircuit by external driving means such as a vibrator, a motor or an electronic pulsing circuit. Here, as soon as the voltage across the primaryof transformer 16 reaches one polarity, in this case the switch 24having closed and passed through various steps, terminal 311 will benegative with respect to terminal 14 and terminal 31 will be drivenpositive by auto transformer action. This voltage is applied across theforward direction of rectifier 67 through coil 47 of the magnetic switchand a coil 41 of saturable reactor 51. The saturable reactor of courserepresents initially a high impedance in this circuit during the timerequired to saturate its core. As soon as the core is saturated, alarger current will flow through coil 47 which will close contacts 25 ofswitch 27 to repeat the sequence of events in exactly the same way asdescribed for switch 26. A certain frequency control is provided by thepie-excitation current through coils 52 and 53 and the variableresistance 58 which will act as the frequency control means.

The same general concepts may also be applied to a recently developedpush-pull magnetic switch for which a typical circuit is shown in Figure2 wherein elements similar to those of Figure 1 are similarly numbered.in Figure 2 the direct-current voltage is applied across terminals 111)and 112 through one of the reactors 134 and 135, through switch contacts124 and 125, through one side of the primary of transformer 116 andthrough an inductance 118.

Saturable reactors 134 and 135 have the same function in this circuit asin the circuit of Figure 1. However, the magnetic switches now beingpush-pull do not have the return springs such as 70 and 71 of Figure l.The function of these return springs is performed by coils 182 and 183with a bucking field provided by coils 181) and 181. The reverseswitching action is accomplished by the voltage across the primary oftransformer 116 with the polarity of rectifiers 184 and 185 determiningwhich coil will be actuated. The R. C. networks 136 and 187 provide afiltering action. In this circuit an alternate connection to thetransformer 116 is provided at terminals 188 and 189 with the samepolarities but reduced voltage.

In the push-pull versions of the magnetic switches the contact springmay be eliminated entirely or have only a very weak force to act as aguide for armature movement. The break action of this switch is providedby the force of the coils 182 and 183. To provide a faster and morepositive closing action of the switch, the flux in coils 182, 183 isbucked out by the activating current in coils 180,181.

With this push-pull magnetic switch the holding coil 42 in Figure l isno longer necessary since there is negligible force against thecontacts. The holding coil is therefore eliminated, or replaced to someextent by additional turns of the series holding coil 123. The coil 182replacing the return spring may have its own bucking coil in series withthe closing coil as noted to improve the response of the switch. Theother functions of the switch and of the circuit are similar to thosedescribed in Figure 1.

While the preferred embodiments of a timing mechanism and theinterconnection of a magnetic switch in an inverter circuit have beenshown, variations of these circuits will be obvious to anyone skilled inthe art.

Having thus described my invention, what is claimed 1. A direct-currentto alternating-current converter comprising; a source of direct current,a transformer having a primary Winding, a first magnetic switch havingcontacts connecting said direct current through said winding in onedirection, a second magnetic switch having contacts connecting saiddirect current through said winding in the other direction, a firstsaturable reactor, a first rectifier, and a first actuating coil of saidfirst magnetic switch connected in series across said winding, saidrectifier having a first polarity with respect to said winding, a secondsaturable reactor, a second rectifier, and a first actuating coil ofsaid second magnetic switch connected in series across said winding,said second rectifier having the other polarity with respect to saidwindmg.

2. A circuit as in claim 1 wherein said first magnetic switch has asecond coil connected in series with said switch contacts producting aholding force for said switch and said second magnetic switch has asecond coil connected in series with said switch contacts to produce aholding force for said second switch.

3. In a converter as in claim 1, a third saturable reactor connected inseries with said first magnetic switch and a fourth saturable reactorconnected in series with said second magnetic switch.

4. In a converter as in claim 1, a third coil of said first magneticswitch, a third rectifier connected in series with said third coil ofsaid first magnetic switch across said transformer winding, a third coilof said second magnetic switch, a fourth rectifier connected in serieswith said third coil of said second magnetic switch across saidtransformer winding, said fourth rectifier having said first polaritywith respect to said transformer winding and said third rectifier havingthe other polarity with respect to said winding.

5. In a converter as in claim 1, said first and second saturablereactors each having a second coil, a variable impedance, said variableimpedance connected in series with said second coil of said firstsaturable reactor and said second coil of said second saturable reactoracross said source of direct current 6. A converter as in claim 1wherein said transformer comprises primary and secondary windings, acondenser and a load impedance connected across the secondary of saidtransformer.

7. A direct-current to alternating-current converter comprising a sourceof direct current, a first magnetic switch having electrical contactsand a plurality of actuating coils, 'a second magnetic switch havingelectrical contacts and a plurality of actuating coils, a transformerhaving primary and secondary windings, a first saturable reactor, afirst rectifier, and a first coil of said first magnetic switchconnected in series across a first transformer primary winding, saidrectifier oriented in a first polarity with respect to said winding,said first coil connected to close the contacts of said magnetic switch,a second saturable reactor, a second rectifier, and a first coil of saidsecond magnetic switch connected in series across said first transformerprimary winding, said second rectifier oriented in the other polaritywith respect to said winding, said first coil of said second magneticswitch connected to close the contacts of said second magnetic switch, athird rectifier connected in series with a seond coil of said firstmagnetic switch across said first winding, a fourth rectifier connectedin series with a second coil of said second magnetic switch across saidwinding, said fourth rectifier having said first polarity with respectto said winding, said second coil of said second magnetic switchpositioned to open the contacts of said second magnetic switch, saidthird rectifier having the other polarity with respect to said windingand said second coil of said first magnetic switch positioned to openthe contacts of said magnetic switch.

8. In a converter as in claim 7, a second coil on said first saturablereactor, a second coil on said second saturable reactor and a variableimpedance connected in series with said second coil of said firstsaturable reactor and said second coil of said second saturable reactoracross said source of direct current.

9. In a converter as in claim 8, an inductance connected in series withsaid variable impedance, said second coil in said first saturablereactor and said second coil of said second saturable reactor acrosssaid source of direct current.

10. In a converter as in claim 7, a third coil of said first magneticswitch connected in series with the contacts of said first magneticswitch and a third coil of said second magnetic switch connected inseries with the contacts of said magnetic switch, said third coilsoriented to hold said contacts in a closed position.

References Cited in the file of this patent UNITED STATES PATENTS2,066,995 Morack Jan. 5, 1937 2,284,794 Bedford June 2, 1942 2,502,932Diebold Apr. 4, 1950 FOREIGN PATENTS 366,259 France Oct. 6, 1906 905,953France Dec. 19, 1945

